docs: move documentation from README to a docs/ folder rendered on codetech.pt

[jfrosorio]

Problem

The README is 578 lines with ~15 usage sections and keeps growing with every feature (Credit Card support is next on the roadmap). It has outgrown the single-file format.

Proposal (repo side)

Follow the Spatie packages pattern:

• Move all usage documentation into a docs/ folder in this repo, one markdown file per topic.
• Each page carries frontmatter with title and weight (sidebar label + order), matching the convention used by spatie/laravel-medialibrary.
• Add a docs/index.md whose frontmatter holds section metadata (title, slogan, githubUrl, branch).
• Shrink the README to: banner, badges, a ~20-line quick start, a link to the docs on codetech.pt/open-source/laravel-eupago, and the community sections (contributing / security / license).
• Docs keep being updated in the same PR as the feature they document.
• The site documents the current major only; older majors keep their frozen READMEs on their X.x maintenance branches.

Suggestion (site side — needs more discussion)

How codetech.pt fetches and renders the docs/ folder is not settled by this issue; the following is only a starting point for discussion:

• Mirror spatie.be's architecture: a scheduled docs:import artisan command that syncs the repo's docs/ folder to local storage (markdown) and public/ (assets), with the site rendering from local disk — never fetching GitHub at request time.
• Possible simplification over Spatie's git sparse-checkout: download the branch tarball (archive/refs/heads/master.tar.gz) with a single request and extract docs/.
• No webhook initially; a scheduled full re-import (Spatie currently runs one daily) is likely enough.

Alternatives (per-request fetch with caching, webhook-driven imports, etc.) are still on the table.

Scope of this issue

Only the repo-side changes: docs/ content structure and the slimmed README. Site-side rendering/import work belongs to the codetech.pt codebase.